Sample clock frequency offset (SCFO) Resolution Team 3 (RT-3) investigation

Abstract

This Resolution Team 3 (RT-3) report contains the results of the investigation of key aspects of the proposed Sample Clock Frequency Offset (SCFO) scheme for the Mid SKA1 telescope. This is a scheme, first proposed by one author (Carlson) at a meeting at the SKAO 25-Jan-2013, to digitize the analog signal at each antenna at a slightly different sample rate and transmit the data to the CSP Mid.CBF for subsequent digital re-sampling to a common sample clock frequency before channelization, correlation, and beamforming. The primary purpose for doing this is to cause de-correlation of sample clock-related self-interference to be able to improve correlated and beamformed signal quality. This report includes an investigation of the efficacy of the method, investigation of its implementation by SADT, DISH, and CSP, presentation of further supporting modeling results augmenting the original modeling work, a note on expansion to SKA-2, as well as possible draw-backs and concerns. Finally, there are potential additional benefits in signal quality in implementing the SCFO scheme, in particular de-correlation of aliased RFI (particularly for Nyquist Zone-2 digitized signals) as well as relaxation of signal-chain anti-aliasing filters transition band roll-off and reject band attenuation prior to digitization in the antenna.